High voltage switching devices and process for forming same

ABSTRACT

The present invention relates to various switching device structures including Schottky diode, P-N diode, and P-I-N diode, which are characterized by low defect density, low crack density, low pit density and sufficient thickness (&gt;2.5 um) GaN layers of low dopant concentration (&lt;1E16 cm −3 ) grown on a conductive GaN layer. The devices enable substantially higher breakdown voltage on hetero-epitaxial substrates (&lt;2 KV) and extremely high breakdown voltage on homo-epitaxial substrates (&gt;2 KV).

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 12/852,223filed on Aug. 6, 2010, and issued as U.S. Pat. No. 8,174,089 on May 8,2012, which in turn is a divisional of U.S. patent application Ser. No.10/513,009 filed on Oct. 27, 2004 and entitled “HIGH VOLTAGE SWITCHINGDEVICES AND PROCESS FOR FORMING SAME,” and issued as U.S. Pat. No.7,795,707 on Sep. 14, 2010, which was filed under the provisions of 35U.S.C. §371 based on International Patent Application No. PCT/US03/13162filed on Apr. 30, 2003, which in turn claims priority to U.S.Provisional Application No. 60/376,629 filed on Apr. 30, 2002 for“SCHOTTKY DIODE STRUCTURE AND MOVPE PROCESS FOR FORMING SAME.” Thedisclosures of each of the foregoing applications and patents are herebyincorporated by reference herein in their respective entireties, for allpurposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to various switching devices of highbreakdown voltage and a process for forming same.

2. Description of the Related Art

By way of background to the present invention, the disclosures of thefollowing documents are hereby incorporated by reference, in theirrespective entireties:

-   Brandić et al., “High Voltage (450 V) GaN Schottky Rectifiers,”    Appl. Phys. Lett., Vol. 74, No. 9, pp. 1266-1268 (Mar. 1, 1999).-   Trivedi et al., “Performance Evaluation of High-Power Wide Band-Gap    Semiconductor Rectifiers,” J. Appl. Phys., Vol. 85, No. 9, pp.    6889-6897 (May 1, 1999).-   U.S. Pat. No. 6,156,581 issued Dec. 5, 2000 in the names of    Robert P. Vaudo, et al. for “GaN-BASED DEVICES USING THICK (Ga, Al,    In)N BASE LAYERS.”-   U.S. Pat. No. 6,440,823 issued Aug. 27, 2002 in the names of    Robert P. Vaudo, et al. for “LOW DEFECT DENSITY (Ga, Al, In)N AND    HVPE PROCESS FOR MAKING SAME.”-   U.S. Pat. No. 6,447,604 issued Sep. 10, 2002 in the names of    Jeffrey S. Flynn et al. for “METHOD FOR ACHIEVING IMPROVED EPITAXY    QUALITY (SURFACE TEXTURE AND DEFECT DENSITY) ON FREE-STANDING    (ALUMINUM, INDIUM, GALLIUM) NITRIDE ((AL, IN, GA)N) SUBSTRATES FOR    OPTO-ELECTRONIC AND ELECTRONIC DEVICES.”-   (Ga, Al, In)N-based materials, which are generically referred to as    “GaN” throughout the description of the present invention    hereinafter unless specified otherwise, is a promising group of    semiconductor materials for fabricating high voltage, high power    microelectronic switching devices, which include, but are not    limited to, Schottky diode rectifiers, P-N diodes, P-I-N diodes,    thyristors with P-N-P-N junctions, and Impact Ionization Avalanche    Transit Time devices (IMPATTs) with N⁺-P-I-P⁺ junctions, etc.

As shown in Table 1, GaN has a number of fundamental properties thatmake it advantageous for use in high power switching applications. Thewide band gap of GaN gives it a high theoretical breakdown field,comparable to 4H—SiC. In addition, GaN has a higher electron mobilityand maximum velocity than 4H—SiC. The thermal conductivity of GaN, whilelower than 4H—SiC, is comparable to that of Si, which is currently themost common material used to fabricate high power switching devices.

TABLE 1 300K PROPERTIES OF CANDIDATE MATERIALS Si 4H—SiC GaN Bandgap(eV) 1.1 3.3 3.4 E_(c), Breakdown field (10⁵ V/cm) 2 30 50*  μ, Electronmobility (cm²/Vs) 1400 800 900   V, Maximum velocity (10⁷ cm/s) 1 2 3 Thermal conductivity (W/cm K) 1.5 4.9 1.7 *theoretical maximum value

Thus, the thicker a semiconductor layer and the lower the dopantconcentration in such semiconductor layer, the higher the breakdownvoltage of the switching device fabricated by using such semiconductorlayer. Therefore, thick, low-doped epitaxial semiconductor layers arerequired in order to fabricate switching devices that will support highbreakdown voltage.

For obtaining a sufficiently high breakdown voltage, the thickness anddoping requirements for GaN layers are less than those for Si or SiClayers. Specifically, FIG. 1 is a plot of the predicted doping andthickness requirements for GaN-based rectifiers. For example, in orderto fabricate a rectifier with a 5 kV reverse breakdown voltage, anapproximately 20 μm thick GaN layer with a background dopingconcentration of n=1×10¹⁶ atoms/cm³ is required. AlGaN alloys, whichhave even larger band gap (6.2 eV max) and higher theoretical breakdownvoltage than those of simple GaN material, enable fabrication ofrectifiers and other switching devices of even higher breakdownvoltages.

In order to fabricate the GaN-based switching devices of high breakdownvoltages, as described hereinabove, it is necessary to deposit thethick, low-doped GaN semiconductor layer of required thickness andbackground doping concentration on top of a highly conductive GaN baselayer that is required for ohmic contact.

However, GaN is difficult to deposit to a thickness greater than acouple of microns on hetero-epitaxial substrates, due to high thermalcoefficient of expansion (TCE) mismatch and formation of threadingdislocations (TDs) and other defects. Novel growth methods, structures,and/or substrates therefore need to be employed to deposit GaN layers toa suitable thickness, as required for fabrication of an electronicdevice. In addition, the epitaxial layers need to be deposited on asubstrate of suitable size, with high uniformity and quality, and withan appropriate configuration of the epitaxial structure (e.g., lateralor vertical) and orientation (e.g., c-plane, r-plane, m-plane, off-axis,on-axis, and offcut direction and angle), so as to meet the cost, yieldand performance needs of the specific device applications.

Currently, Si, sapphire, SiC, HVPE/sapphire, and free-standing bulk GaNsubstrates are available in various sizes and configurations that suitthe needs of various high voltage diode applications. Typically, lowcost, low power (<1 kV) devices employ a hetero-epitaxial substrate suchas Si and sapphire, while high cost, high power (>1 kV) devices usebetter lattice matched substrates, such as SiC, HVPE/sapphire, andfree-standing bulk GaN. Provision of suitable epitaxy quality onhetero-epitaxial substrates is difficult, due to the differences inthermal expansion coefficients and lattice mismatches between thehetero-epitaxial substrates and the GaN layers grown thereon, whichresult in high dislocation defect density and severe cracking of the GaNepitaxial layers. Growth of epitaxial GaN layer on GaN or HVPE/sapphiresubstrates are less affected by the TCE and lattice mismatches, butother problems, such as interface charge elimination between the GaNsubstrates and the epitaxial layer, may still need to be overcome. Inall cases, problems with cracking are exacerbated when GaN epitaxiallayer is doped with Si to form the highly conductive n-type GaN layer inhigh breakdown voltage devices.

It is therefore an object of the present invention to provide a highquality and uniform MOVPE epitaxial layer of large diameter on asuitable hetero-epitaxial or homo-epitaxial substrate with low crackingdensity, low pitting density, and high n-layer conductivity, upon whicha thick, low-doped GaN layer can be formed for fabricating GaN-basedswitching devices with high breakdown voltages.

SUMMARY OF INVENTION

The present invention in one aspect relates to a high voltage breakdowndevice with good current spreading fabricated on a hetero-epitaxialsubstrate, such as a sapphire substrate or a SiC or Si substrate of highvertical conductivity. Severe cracking is generally observed inepitaxial GaN layers formed on such hetero-epitaxial substrate, whichmay be partially suppressed, but not entirely eliminated, by providinghigh doping levels (>5E18 cm⁻³ and <3E19 cm⁻³) or delta doping in suchepitaxial GaN layers.

One embodiment of the present invention therefore employs two highlyconductive GaN layers, one of which has a relatively higher dopingconcentration and the other of which has a relatively lower dopingconcentration, for further suppression of the cracking in the undopedepitaxial GaN layers subsequently formed thereon.

Another embodiment of the present invention provides an undoped GaNlayer underneath the highly conductive GaN layer, which functions toimprove material quality and reduce pitting and cracking in the undopedepitaxial GaN layers subsequently formed on such highly conductive GaNlayer.

Still another embodiment of the present invention utilizesstrain-reducing dopant materials, such as germanium, in place of theconventional Si dopant used for n-type doping of the conductive GaNlayer. Since germanium fits in the Ga site better than Si, doping of theconductive GaN layer with germanium significantly reduces crackingtherein.

The present invention in another aspect relates to a high voltagebreakdown device fabricated on a free-standing homo-epitaxial GaNsubstrate, or a HVPE/sapphire base structure.

The term “HVPE/sapphire base structure” in the present invention refersto a base structure comprising a device quality, crack free GaN baselayer of about 10 μm in thickness fabricated on a sapphire substrate viathe hydride vapor phase epitaxy (HVPE) process, as described in U.S.Pat. No. 6,156,581 issued Dec. 5, 2000 in the names of Robert P. Vaudoet al. for “GaN-BASED DEVICES USING THICK (Ga, Al, In)N BASE LAYERS,”the content of which has been incorporated herein by reference in itsentirety for all purposes.

In one embodiment of the present invention, the free-standing GaNsubstrate or the HVPE/sapphire base structure comprises an undoped GaNtop layer, and subsequent epitaxial growth of GaN layer thereupon iscarried out uniformly by eliminating dopant or conductivity at theinterface of the epitaxial GaN layer and the substrate or basestructure. The quality and performance of such epitaxial GaN layer canbe further improved by employing alternative growth orientations, offcutangles, and offcut directions, as described in U.S. Pat. No. 6,447,604issued Sep. 10, 2002 in the names of Jeffrey S. Flynn et al. for “METHODFOR ACHIEVING IMPROVED EPITAXY QUALITY (SURFACE TEXTURE AND DEFECTDENSITY) ON FREE-STANDING (ALUMINUM, INDIUM, GALLIUM) NITRIDE ((AL, IN,GA)N) SUBSTRATES FOR OPTO-ELECTRONIC AND ELECTRONIC DEVICES,” thecontent of which is incorporated herein by reference in its entirety forall purposes.

The present invention in a further aspect relates to a microelectronicdevice that comprises:

-   -   (a) a first conductive GaN interfacial layer having a top        surface that is characterized by a dislocation defect density of        not more than about 5×10⁶/cm²;    -   (b) a second GaN layer having a dopant concentration of not more        than about 1×10¹⁶/cm³, formed over the top layer of said first        conductive GaN base layer; and    -   (c) at least one metal contact over said first GaN layer,        forming a metal-to-semiconductor junction therewith.

Note that the term “GaN” as used in the present invention, unlessspecified otherwise, broadly covers any Al_(x)In_(y)Ga_((1-x-y))N-basedmaterials, which include, but are not limited to, GaN, Al_(x)Ga_(1-x)N,Al_(x)In_(y)Ga_(1-x-y)N, In_(y)Ga_(1-y)N, etc.

The unit for dislocation defect density refers to the number ofdislocation defects measured per square centimeter.

The unit for dopant concentration refers to the number of dopant atomsmeasured per cubic centimeter.

Such microelectronic device is preferably a Schottky diode rectifier,having a Schottky contact and an Ohmic contact.

Another aspect of the present invention relates to a microelectronicdevice, which comprises:

-   -   (a) a foreign substrate;    -   (b) a nucleation buffer layer overlying said foreign substrate;    -   (c) a first GaN layer overlying said nucleation buffer layer,        said first GaN layer having a dopant concentration of not more        than about 1×10¹⁶/cm³;    -   (d) a second, conductive GaN layer overlying said first GaN        layer;    -   (e) a third GaN layer overlying said second, conductive GaN        layer, said third GaN layer having a dopant concentration of not        more than about 1×10¹⁶/cm³; and    -   (f) at least one metal contact over said third GaN layer,        forming a metal-to-semiconductor junction therewith.

Such microelectronic device is also preferably a Schottky dioderectifier, having a Schottky contact and an Ohmic contact.

Still another aspect of the present invention relates to amicroelectronic device structure having:

-   -   (a) a first GaN layer of n-type conductivity, having a top        surface characterized by a dislocation defect density of not        more than about 5×10⁶/cm²;    -   (b) a second GaN layer having a dopant concentration of not more        than about 1×10¹⁵/cm³, formed over the top layer of said first        GaN layer;    -   (c) a third GaN layer of p-type conductivity, formed over said        second GaN layer; and    -   (d) at least one metal contact overlying said third GaN layer.

Such microelectronic device structure is preferably a P-I-N diode,having at least two Ohmic contacts, which include one p-type contact andone n-type contact.

Other aspects, features and embodiments of the invention will be morefully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of the predicted doping concentration and thicknessrequirements for GaN-based rectifiers.

FIG. 2A is a schematic view of a GaN-based mesa Schottky dioderectifier, according to one embodiment of the present invention.

FIG. 2B is a schematic view of a GaN-based planar Schottky dioderectifier, according to one embodiment of the present invention.

FIG. 3 is the I-V curve for the GaN-based mesa Schottky diode rectifierof FIG. 2A.

FIG. 4 shows the scanning electron microscopic view of a Schottkycontact of the GaN-based mesa Schottky diode rectifier of FIG. 2A afterelectric breakdown.

FIG. 5 shows a free-standing GaN-based Schottky rectifier devoid offoreign substrate, according to one embodiment of the present invention.

FIG. 6A shows a generic view of a group of GaN-based mesa Schottkyrectifiers, according to one embodiment of the present invention.

FIG. 6B shows a generic view of a group of GaN-based planar Schottkyrectifiers, according to one embodiment of the present invention.

FIG. 7A shows a 32.5× Nomarski view of the center of the nitridematerial for forming a GaN-based Schottky rectifier, according to oneembodiment of the present invention.

FIG. 7B shows a 32.5× Nomarski view of the edge of the nitride materialfor forming a GaN-based Schottky rectifier of FIG. 7A.

FIG. 8A shows a 32.5× Nomarski view of the center of the nitridematerial for forming a GaN-based Schottky rectifier, according to oneembodiment of the present invention.

FIG. 8B shows a 32.5× Nomarski view of the edge of the nitride materialfor forming a GaN-based Schottky rectifier of FIG. 8A.

FIG. 9A shows a 32.5× Nomarski view of the center of the nitridematerial for forming a GaN-based Schottky rectifier, according to oneembodiment of the present invention.

FIG. 9B shows a 32.5× Nomarski view of the edge of the nitride materialfor forming a GaN-based Schottky rectifier of FIG. 9A.

FIG. 10 shows a schematic view of a GaN-based mesa Schottky rectifier,according to one embodiment of the present invention.

FIG. 11 shows a schematic view of a GaN-based P-I-N diode, according toone embodiment of the present invention.

FIG. 12 shows the I-V curve for the GaN-based P-I-N diode of FIG. 11.

FIG. 13 shows a schematic view of a free-standing, GaN-based P-I-Ndiode, according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF

For fabricating microelectronic switching devices of relatively lowbreakdown voltages (i.e., <2 kV), thin GaN layers can be directlydeposited by MOVPE on the foreign substrate, such as sapphire, Si, andSiC. Despite the fact that such directly deposited GaN layers arerelatively thin (i.e., <10 μm), strain in the GaN layer caused bythermal expansion differences between the foreign substrate and the GaNlayers results in significant cracking, pitting and defect production.It is therefore difficult to deposit low-doped GaN layers of thisthickness (i.e., <10 μm) on top of thin, conductive GaN layers onforeign substrates for fabricating Schottky rectifiers having breakdownvoltages lower than about 2 kV. The following innovative and preferredembodiments address these limitations.

Conductive GaN Base Layer Formed Over One or More Interfacial Layers byMOVPE

The present invention provides a conductive GaN base layer of n-typeconductivity, which is formed over a foreign substrate, with one or moreinterfacial layers therebetween for reducing defect density of suchconductive GaN base layer.

An Al-containing nucleation buffer layer is first provided on theforeign substrate, prior to the formation of the conductive GaN baselayer to ensure proper nucleation of such conductive GaN base layer,since silicon and other impurities used as n-type dopants can interruptthe nucleation and coalescence process during the heteroepitaxialgrowth.

A thin (i.e., ≈0.1 μm), low-doped (i.e., dopant concentration of notmore than 1×10¹⁶/cm³) GaN layer can be deposited on top of suchnucleation buffer layer, prior to the formation of the conductive GaNbase layer, to further improve the nucleation result.

FIG. 6A shows a generic view of a mesa-type Schottky diode structure 30with a conductive GaN base layer 34 formed over a foreign substrate 32and having a nucleation buffer layer 42 and a thin, low-doped GaN layer44 therebetween as interfacial layers. A thick, low-doped GaN layer 36is subsequently formed on the conductive GaN base layer 34 with Schottkycontact 38 fabricated thereon, and ohmic contacts 39A and 39B fabricatedon the conductive GaN base layer 34.

FIG. 6B shows a generic view of a planar-type Schottky diode structure30′, having similar structures to those of the mesa-type Schottky diode30 as shown in FIG. 6A, except that the ohmic contacts 39A′ and 39B′ ofthe planar Schottky diode 30′ are formed directly on the conductive GaNbase layer 36′.

For Schottky diode structures with recessed contacts as shown in FIG.6A, it is advantageous to have a thick, conductive GaN layer 34 (whichis doped with Si) for high lateral conductivity and current spreading,low contact resistance and good ohmic contacts, as well as for ease ofetching from the top of the structure to prevent undershoot andovershoot of the conductive GaN layer 34 in the etching process.Improved ohmic contacts and conductive GaN layer 34 will improve the I-Vcharacteristics of the Schottky diode. For example, the forward turn-onresistance (slope of forward I-V curve) will be increased with areduction in resistivity or contact resistance in layer 34. Otherbenefits in the device can be effectuated by modifying the region of theconductive base layer 34 in closest proximity to the low-doped GaN layer36. For example, the doping level near the low-doped layer 36 can bedesigned to produce a desired depletion in the low-doped GaN layer 36.Furthermore, the quality, doping level and defect density of thelow-doped layer can be modified to improve the reverse I-Vcharacteristics of such Schottky diode, including breakdown voltage andleakage current.

The following examples of Schottky diodes, which have the genericstructure as shown in FIG. 6A, but with varying layer thickness anddopant concentration, demonstrate the effects of layer thickness anddopant concentration on the quality of such diodes:

Structure A

-   Layer (1)—2.5 μm undoped GaN (or lightly n-type dopant concentration    of not more than 1×10¹⁶/cm³)-   Layer (2)—2.0 μm Si-doped conductive GaN (3×10¹⁹/cm³)-   Layer (3)—0.1 μm undoped GaN (or lightly n-type dopant concentration    of not more than 1×10¹⁶/cm³)-   Layer (4)—Nucleation buffer-   Layer (5)—Sapphire substrate

This Schottky diode structure A exhibits high pitting and crackingdensity.

Structure B

-   Layer (1)—2.5 μm undoped GaN (or lightly n-type dopant concentration    of not more than 1×10¹⁶/cm³)-   Layer (2)—1.0 μm Si-doped conductive GaN (1×10¹⁹/cm³)-   Layer (3)—0.1 μm undoped GaN (or lightly n-type dopant concentration    of not more than 1×10¹⁶/cm³)-   Layer (4)—Nucleation buffer-   Layer (5)—Sapphire substrate

This Schottky diode structure B has a conductive GaN layer of reducedthickness and dopant concentration, in comparison with that of StructureA. Severe cracking and pitting is still observed in this Schottky diodestructure B, as shown in FIGS. 7A and 7B.

Structure C

-   Layer (1)—2.5 μm undoped GaN (or lightly n-type dopant concentration    of not more than 1×10¹⁶/cm³)-   Layer (2)—2.0 μm Si-doped conductive GaN (1.5×10¹⁹/cm³)-   Layer (3)—0.6 μm undoped GaN (or lightly n-type dopant concentration    of not more than 1×10¹⁶/cm³)-   Layer (4)—Nucleation buffer-   Layer (5)—Sapphire substrate

This Schottky diode structure C has an undoped or low-doped GaNinterfacial layer of increased thickness beneath the conductive GaNlayer, in comparison with that of Structure A. Reduced pitting densityis observed in this Schottky diode structure C, as shown in FIGS. 8A and8B.

Structure D

-   Layer (1)—2.5 μm undoped GaN (or lightly n-type dopant concentration    of not more than 1×10¹⁶/cm³)-   Layer (2)—0.5 μm Si-doped conductive GaN (1.5×10¹⁹/cm³)-   Layer (3)—0.6 μm undoped GaN (or lightly n-type dopant concentration    of not more than 1×10¹⁶/cm³)-   Layer (4)—Nucleation buffer-   Layer (5)—Sapphire substrate

This Schottky diode structure D has a conductive GaN layer of reducedthickness and dopant concentration, and an undoped or low-doped GaNinterfacial layer of increased thickness beneath the conductive GaNlayer, in comparison with those of Structure A. No significant crackingor pitting is observed in such Schottky diode structure D, as shown inFIGS. 9A and 9B.

A composite structure E shown as follows can be devised to providereduced cracking and pitting quality, while still maintaining lowresistivity of the device.

Structure E

-   Layer (1)—2.5 μm undoped GaN (or lightly n-type dopant concentration    of not more than 1×10¹⁶/cm³)-   Layer (2)—0.1 μm Si-doped conductive GaN sub-layer (1.5×10¹⁹/cm³)-   Layer (3)—1.9 μm Si-doped conductive GaN sub-layer (2×10¹⁸/cm³)-   Layer (4)—0.6 μm undoped GaN (or lightly n-type dopant concentration    of not more than 1×10¹⁶/cm³)-   Layer (5)—Nucleation buffer-   Layer (6)—Sapphire substrate

This Structure E is schematically shown in FIG. 10 (as structure 50therein), which comprises a first conductive GaN sub-layer 54A of asmaller thickness and a higher dopant concentration, which is adjacentto the contact-forming undoped or low-doped GaN layer 56, and a secondconductive GaN sub-layer 54B of a bigger thickness and a lower dopantconcentration, which is adjacent to the interfacial undoped or low-dopedGaN layer 44. Such Schottky diode structure exhibits a sheet resistanceof 36 ohm/square, which is close to the 14 ohm/square sheet resistanceof Structure A. Further optimization of the conductive GaN sub-layersand the interfacial undoped or low-doped GaN layer with respect to theirthickness and doping level can be implemented to reach the 14ohm/square, without significantly increasing the cracking or pittingdensity.

In general, placing a thick, undoped or low-doped GaN interfacial layerunderneath the conductive GaN base layer reduces the cracking andpitting in the overall structure. Other interfacial layers or alloys canalso be used for strain relief or thermal expansion coefficient (TCE)relief, so as to further reducing the cracking and pitting in theSchottky diode structure formed and to further improve the devicequality.

The conductive GaN base layers of n-type conductivity are doped by Si inthe above-listed exemplary structures. Alternatively, they can be dopedby germanium (Ge) or other n-type dopants with similar atomic size tothe AlInGaN atoms, to enable modified elasticity, strain or TCE effects.

Delta doping, as more fully described in U.S. Patent ApplicationPublication No. 2003/0178633 published on Sep. 25, 2003 in the names ofJeffrey S. Flynn and George R. Brandes for “DOPED GROUP III-V NITRIDEMATERIALS, AND MICROELECTRONIC DEVICES AND DEVICE PRECURSOR STRUCTURESCOMPRISING SAME,” the disclosure of which hereby is incorporated byreference in its entirety, can also be incorporated into the dopedlayers for providing a thick stack of average low resistivity material,as in the conductive sub-layers 54A and 54B of the Schottky diodestructure 50 in FIG. 10.

Conductive GaN Base Layer Formed by HVPE

For example, we grew an approximately 10 μm thick GaN layer directly ona sapphire substrate by hydride vapor phase epitaxy (HVPE), which wassubsequently used to form a GaN-based Schottky diode, as shownschematically in FIGS. 2A and 2B. The greater thickness are able to beachieved by HVPE compared with MOVPE due to the reduce thermalcoefficient of expansion difference, a much thicker and more heavilydislocated buffer and other interfacial defects which result in overalllower strain in the epi layer.

The GaN-based mesa-type Schottky diode 10 in FIG. 2A comprises asapphire substrate 12, a highly conductive GaN layer 14 of n-typeconductivity is present at the GaN/sapphire interface, upon which anapproximately 10 μm GaN layer 16 of low dopant concentration(≈1×10¹⁶/cm³) is fabricated. Gold is used for forming the Schottkycontact 18, and Ti/Al/Ni/Au is used for forming the ohmic contacts 19Aand 19B. The I-V curve for this GaN-based mesa-type Schottky diode isshown in FIG. 3. The reverse breakdown voltage of such GaN-based mesaSchottky diode was measured, which is about 450V. FIG. 4 shows thescanning electron microscopic view of the Schottky contact 18 of theGaN-based mesa Schottky diode in FIG. 2A, after electric breakdown. Themelted Au at edges indicates premature edge breakdown, which limits theoverall reverse breakdown voltage of such Schottky diode withoutpassivation or use of guard ring or similar steps.

FIG. 2B shows a GaN-based planar Schottky diode 10′, having structuressimilar to those of the mesa diode 10 as shown in FIG. 2A, except thatthe ohmic contacts 19A′ and 19B′ (along with Schottky contact 18′) ofsuch planar Schottky diode 10′ are formed on the 10 μm GaN layer 16′ oflow dopant concentration instead of on the highly conductive GaN layer14′ that is disposed over the sapphire substrate 12′.

The GaN-based Schottky diodes shown in FIGS. 2A and 2B are thereforeonly suitable for switching applications at relatively low voltages(i.e., <2 kV). However, many industrial applications require switchingdevices operable at higher voltages (≧2 kV). It is therefore anotherobject of the present invention to provide a GaN-based Schottky diodewith breakdown voltages higher than about 2 kV.

In order to provide high voltage GaN-based switching devices, it isnecessary to provide GaN layers of increased thickness and lower dopantconcentration, as shown by the prediction plot of FIG. 1. For GaN layersgrown on foreign substrate by MOVPE, the lattice mismatch and thedifferences in thermal expansion coefficient between the foreignsubstrate and the GaN layer grown thereon result in high dislocationdefect density in such GaN layer with high level of strain. When thethickness of such GaN layer is increased substantially, the strain cancause severe cracking in such GaN layer, rendering it unsuitable fordevice fabrication.

The present invention also provides a new Schottky diode structure,which comprises a conductive GaN base layer having a top surface of verylow dislocation defect density (i.e., ≦5×10⁶/cm²), upon which alow-doped GaN layer having a dopant concentration of not more than about1×10¹⁶/cm³ can be grown. Because the conductive GaN base layer has a topsurface of low dislocation defect density, strain-relaxed, low-doped GaNlayer thereon can be grown to a sufficient thickness (i.e., ≧10 μm)without cracking, which can be subsequently used for fabricating aswitching device of high breakdown voltage.

Free-Standing Conductive GaN Base Layer

A thick, conductive GaN layer of a low dislocation defect density (i.e.,≦5×10⁶/cm²) can be first grown on a foreign substrate (such as sapphire,Si, or SiC) by hydride vapor phase epitaxy (HVPE) at decreased growthtemperature (i.e., from about 985° C. to about 1010° C.). At suchdecreased HVPE growth temperature, the GaN layer suffers from lessstrain induced by the thermal expansion coefficient differences betweenthe foreign substrate and the GaN layer, which results in reduceddislocation defect density, as described in U.S. Pat. No. 6,440,823issued on Aug. 27, 2002 for “LOW DEFECT DENSITY (Ga, Al, In)N AND HVPEPROCESS FOR MAKING SAME,” the content of which is incorporated byreference herein in its entirety for all purposes. Such decreased HVPEgrowth temperature also increases the n-type conductivity of the GaNlayer so formed, and therefore can be used to form conductive GaN layerof n-type conductivity. The thick, conductive GaN layer of a lowdislocation defect density can be separated from the foreign substrateto produce a free-standing conductive GaN base layer.

Further, the epitaxy quality of such conductive GaN layer can be furtherimproved by various techniques described in U.S. Pat. No. 6,447,604issued Sep. 10, 2002 in the names of Jeffrey S. Flynn et al. for “METHODFOR ACHIEVING IMPROVED EPITAXY QUALITY (SURFACE TEXTURE AND DEFECTDENSITY) ON FREE-STANDING (ALUMINUM, INDIUM, GALLIUM) NITRIDE ((AL, IN,GA)N) SUBSTRATES FOR OPTO-ELECTRONIC AND ELECTRONIC DEVICES,” thecontent of which is incorporated by reference herein in its entirety forall purposes. Low temperature interfacial layers, alternative crystalorientations (e.g., m-plane, r-plane, c-plane), and various offcutangles and directions are preferably employed to modify crystal growthquality, annihilate defects, modify point defect density, modifyimpurity incorporation, change crystal polarization, modify crystalmobility, increase breakdown voltage, reduce leakage current, etc., asdescribed in the U.S. Pat. No. 6,447,604, for further improvement of theepitaxy quality of such conductive GaN layer and the performance of highbreakdown voltage devices.

The free-standing GaN substrate can be usefully employed to provide aninitial undoped GaN layer, the growth of which can then be continued ina MOVPE growth process for further increase of thickness, reduction ofdislocation density, and improvement of the breakdown voltage. For suchcontinued growth of the undoped GaN layer through MOVPE, it is importantthat the electrically active impurities and defects are reduced at thegrowth interface, by appropriately controlling the cleaning of thesubstrate, the heat-up conditions, and nucleation on the substrate, asdescribed in the U.S. Pat. No. 6,447,604.

FIG. 5 shows a schematic view of a high voltage Schottky rectifier 20,according to one embodiment of the present invention. Such Schottkyrectifier 20 comprises a free-standing, conductive GaN base layer 22having a thickness of greater than about 50 μm and a top surfacecharacterized by a low dislocation defect density of not more than about5×10⁶/cm². A low-doped GaN layer 24 grown thereupon, which ischaracterized by a dopant concentration of not more than about1×10¹⁶/cm³ and a thickness of greater than about 10 μm. Such Schottkyrectifier 20 is devoid of any foreign substrate, so the Schottky contact26 can be formed over the low-doped GaN layer 24 at one side, and theohmic contact 28 can be formed over the free-standing, conductive GaNbase layer 22 at the opposite side.

GaN-Based P-N and P-I-N Diode Structures

GaN-based P-N and P-I-N diodes with high breakdown voltages are also ofinterest for high power device applications. The ability to fabricate PNor P-I-N junctions with high breakdown voltages is a key step toward thedevelopment of power devices such as thyristors and IMPATTs.

The present invention in one aspect provides GaN P-I-N diodes withbreakdown voltages of about 320V and 450V, which were fabricated bygrowing the GaN-based P and I layers by MOVPE on an HVPE GaN layer witha highly conductive n-type GaN layer near the epilayer/substrateinterface. Alternative P-N and P-I-N structures including AlGaN P-I-Nstructures and the use of GaN as the substrate material for the epitaxyand device are also contemplated by the present invention.

A schematic GaN P-I-N diode structure 70 is shown in FIG. 11, whichcomprises an approximately 10 μm thick GaN layer of n-type conductivitygrown by HVPE on a sapphire substrate 72. Such n-type GaN layer can beviewed as further comprising a higher conductive 2 μm GaN sub-layer 74and a less conductive 8 μm GaN sub-layer 76 having a conductivity ofabout 1×10¹⁶/cm³, which functions as the N-junction. An approximately0.5 μm low-doped GaN layer 77 is subsequently grown on the GaN sub-layer76 by MOVPE, under conditions which result in a background dopantconcentration of less than 1×10¹⁵/cm³, which functions as theI-junction. An approximately 0.5 μm p-type GaN layer 78 is grown on thelow-doped GaN layer 77 by MOVPE, under conditions which result in a holeconcentration of about 1×10¹⁷/cm³, which functions as the P-junction.P-type ohmic contact 79A and n-type ohmic contacts 79B and 79C aresubsequently formed, so as to provide a complete P-I-N diode.

P-I-N diode structure of this type is formed by reactive ion etching, toprovide a mesa-structure as that shown in FIG. 11, and standardmetallization procedures to provide the p-type and n-type ohmiccontacts. The I-V curve for such P-I-N diode structure is shown in FIG.12. A breakdown voltage of approximately 320V was measured in this P-I-Ndevice. A breakdown voltage of 450V was obtained in another P-I-N diodeof similar structure. In both cases, the devices exhibited prematurebreakdown at the corners and edges, indicating that the device was notlimited by material quality, rather the device design.

P-N or P-I-N diodes of higher breakdown voltages can be fabricated usingimproved edge termination and thicker I-layers. The thickness of the GaNlayers in the P-I-N structures was limited by strain-induced cracking,which has been observed in GaN layers grown on foreign substrate, suchas sapphire, when the thickness of such GaN layers becomes greater thanapproximately 10 μm, as described hereinabove in the section relating toGaN-based Schottky diode structure. The free-standing, low dislocationdefect density GaN layer as described for the Schottky diode structurecan also be used for fabricating high voltage P-N or P-I-N diodestructure.

Specifically, the techniques for producing low dislocation defectdensity, free-standing GaN substrates were disclosed in U.S. Pat. No.6,440,823 issued on Aug. 27, 2002 for “LOW DEFECT DENSITY (Ga, Al, In)NAND HVPE PROCESS FOR MAKING SAME,” U.S. Patent Application PublicationNo. 2001/0008656 in the names of Michael A. Tischler, Thomas F. Kuechand Robert P. Vaudo for “BULK SINGLE CRYSTAL GALLIUM NITRIDE AND METHODOF MAKING THE SAME,” U.S. Pat. No. 5,679,152 issued Oct. 21, 1997; andU.S. Pat. No. 6,156,581 issued Dec. 5, 2000 for “GAN-BASED DEVICES USING(GA, AL, IN)N BASE LAYERS.”

A schematic view of a high voltage P-I-N structure 80 of the presentinvention is shown in FIG. 13, which includes a N-junction formed by afree-standing, conductive GaN base layer 86 of n-type conductivity,characterized by a dislocation defect density of not more than 5×10⁶/cm²and a preferred thickness of more than about 50 μm. Such low dislocationdefect density, free standing GaN base layer is formed by methodssimilar to those described hereinabove for the Schottky diode structure.An I-junction comprising a thick, low-doped GaN layer 87 having a dopantconcentration of not more than 1×10¹⁵/cm³ are formed upon suchconductive GaN base layer 86. The GaN base layer 86 can be removed fromthe foreign substrate upon which it is formed, either prior to or afterformation of the low-doped GaN layer 87. Since the conductive GaN baselayer 86 has a low dislocation defect density, the low-doped GaN layer87 formed thereon can be grown to a sufficient thickness, i.e., morethan 10 μm, for enhancing the overall breakdown voltage of the P-I-Nstructure 80. A P-junction comprising a GaN layer 88 of p-typeconductivity and above 0.25 μm in thickness can be subsequently formedover the low-doped GaN layer 87.

The p-type ohmic contact 89A can be formed over the p-type GaN layer 88,while the n-type ohmic contact 89B can be formed over the n-typeconductive GaN base layer 86, and the thickness of the I-layer 87 can beextended to thickness >10 μm, for increasing the breakdown voltage. Avertical structure as shown in FIG. 13 is advantageous because itminimizes current crowding in the n-type layer, as opposed to thelateral device shown in FIG. 11. Reduced dislocation defect density inthe n-type conductive GaN layer 86 also leads to reduced leakagecurrents in the device.

P-I-N diode structure which contain (Al, Ga)N or (Al, Ga, In)N alloyscan also be fabricated. For example, the use of AlGaN, which has a widerband gap than GaN, may lead to higher break-down voltages and theability to use thinner, low-doped layers in the device structure.

The P-N and P-I-N diode technology described in the present inventioncan be used to fabricate more complex bipolar GaN-based power devicessuch as thyristors (p-n-p-n) and IMPATTs (n⁺-p-i-p⁺).

Although the invention has been variously disclosed herein withreference to illustrative embodiments and features, it will beappreciated that the embodiments and features described hereinabove arenot intended to limit the invention, and that other variations,modifications and other embodiments will suggest themselves to those ofordinary skill in the art. The invention therefore is to be broadlyconstrued, consistent with the claims hereafter set forth.

1. A microelectronic device structure adapted for high voltageoperation, said microelectronic device structure comprising: (a) a firstconductive GaN base layer comprising a GaN structure having a thicknessof greater than about 50 μm and having a top surface characterized by adislocation defect density of not more than about 5×10⁶/cm²; (b) asecond GaN layer having a dopant concentration of not more than about1×10¹⁶/cm³, disposed adjacent to the top surface of said firstconductive GaN base layer; and (c) at least one metal contact adjacentto at least one of (i) the top surface of said first conductive GaN baselayer and (ii) said second GaN layer, forming a metal-to-semiconductorjunction with at least one of said first conductive GaN base layer andsaid second GaN layer; wherein said microelectronic device structure hasa breakdown voltage of at least about 450V.
 2. The microelectronicdevice structure of claim 1, having a breakdown voltage of at least1000V.
 3. The microelectronic device structure of claim 1, having abreakdown voltage of at least 2000V.
 4. A microelectronic devicestructure adapted for high voltage operation, said microelectronicdevice structure comprising: (a) a first GaN layer of n-typeconductivity and comprising a GaN structure having a thickness ofgreater than about 50 μm and having a top surface characterized by adislocation defect density of not more than about 5×10⁶/cm²; (b) asecond GaN layer having a dopant concentration of not more than about1×10¹⁵/cm³, disposed adjacent to said first GaN layer; (c) a third GaNlayer of p-type conductivity, disposed adjacent to said second GaNlayer; and (d) at least one metal contact disposed adjacent to saidthird GaN layer; wherein the microelectronic device structure has abreakdown voltage of at least about 450V.
 5. The microelectronic devicestructure of claim 4, having a breakdown voltage of at least 1000V. 6.The microelectronic device structure of claim 4, having a breakdownvoltage of at least 2000V.
 7. The microelectronic device structure ofclaim 1, wherein a first metal contact of said at least one metalcontact forms a Schottky contact with said second GaN layer, and whereina second metal contact of said at least one metal contact forms an ohmiccontact with said first conductive GaN base layer.
 8. Themicroelectronic device structure of claim 1, wherein the top surface ofsaid first conductive GaN base layer is undoped, and wherein the secondGaN layer is subsequently grown uniformly adjacent to the firstconductive GaN base layer.
 9. The microelectronic device structure ofclaim 1, comprising a Schottky diode selected from the group consistingof mesa-type Schottky diodes and planar-type Schottky diodes.
 10. Themicroelectronic device structure of claim 1, further comprising at leastone electrically conductive underside contact adjacent to the firstconductive GaN base layer.
 11. The microelectronic device structure ofclaim 1, wherein the at least one metal contact forms ametal-to-semiconductor junction with the second GaN layer.
 12. Themicroelectronic device structure of claim 1, wherein the firstconductive GaN base layer is of n-type conductivity.
 13. Themicroelectronic device structure of claim 4, wherein said second GaNlayer is more than about 10 μm in thickness, and wherein said third GaNlayer of p-type conductivity is more than about 0.25 μm in thickness.14. The microelectronic device structure of claim 4, wherein a firstmetal contact of said at least one metal contact forms a first ohmiccontact with said first GaN layer of n-type conductivity, and wherein asecond metal contact of said at least one metal contact forms a secondohmic contact with said third GaN layer of p-type conductivity.
 15. Themicroelectronic device structure of claim 4, wherein the second GaNlayer is doped with germanium.
 16. The microelectronic device structureof claim 4, wherein the top surface of said first GaN layer of n-typeconductivity is undoped, and wherein the second GaN layer issubsequently grown uniformly adjacent to the first GaN layer.
 17. Themicroelectronic device structure of claim 4, wherein the third GaN layeris less than 50 mm in thickness.
 18. The microelectronic devicestructure of claim 4, comprising a Schottky diode selected from thegroup consisting of mesa-type Schottky diodes and planar-type Schottkydiodes.
 19. The microelectronic device structure of claim 4, furthercomprising at least one electrically conductive underside contactadjacent to the first GaN layer.
 20. The microelectronic devicestructure of claim 4, wherein the at least one metal contact is disposedin direct contact with the third GaN layer.